A typical large-scale offshore wind power plant (sometimes referred to as a wind farm) architecture consists of wind turbines, a medium voltage collection system, an offshore substation, a high voltage transmission system and an onshore substation to interface with the main power grid. Medium voltage AC collection systems, typically about 33 kV, are currently used, irrelevant of the type of wind turbine technologies utilized by the wind farm. The choice of transmission system is mainly determined by the distance from the offshore wind power plant to the onshore grid connection point. For close-to-shore wind power plants, high voltage AC (HVAC) transmission systems are used. For offshore wind power plants with long distances to shore, voltage source converter based high voltage DC (VSC-HVDC) systems have proven technically advantageous and cost-effective over conventional HVAC solutions.
In view of system opportunities from wind turbines to the grid connection point, it is possible to improve overall system efficiency and performance by adopting medium voltage DC (MVDC) collection system within the wind power plant. The prerequisite for adopting a MVDC collection system for offshore wind power plant is that medium voltage conversion equipment is used at the generator level, thus allowing elimination of the wind turbine's step-up transformers. Double-fed induction generators have been the prevailing wind generator technology and have dominated the market. However, such technology has become less preferred than other emerging generator technologies such as permanent magnet synchronous generators with full power back-to-back converters due to the fact that the emerging technologies provide better reliability and performance.
As the offshore wind energy infrastructure continues to grow, the electrical system in an offshore wind power plant and its connection to the main power grid pose technical challenges with regard to overall system efficiency and performance. The internal wind power collection system of future large offshore wind power plants may be represented by the following electrical characteristics: 400 MW and higher total capacity; increased power of single wind turbines in the range of 5-10 MW; longer distances between turbines, 1,000-1,500 m; maximum distance between two turbines, 10-30 km; and five to eight turbines or 30-40 MW power level, per feeder. As such, there are several technical challenges for medium voltage collection grids. One problem with a typical 33 kV MVAC collection system is low efficiency resulting from higher power loss of wind turbine transformers and AC cables. Increasing collection system voltage from 33 kV to 72 kV can significantly improve efficiency of the collection system. However, this solution may be restricted by the increased dimension of electrical equipment inside the wind turbine tower, such as wind turbine transformers and switchgear. There are also increased demands for high power density and less complexity in wind turbines and offshore platforms.
MVDC collection systems present a promising solution to overcome the aforementioned problems. The main challenge for MVDC solutions for large offshore wind farms is the need for a high range medium voltage electrical system, up to 40-50 kV or higher, without DC/DC power conversion at the wind turbines. Some solutions provide for configuration of a multi-phase wind turbine generator with cascaded AC/DC converters to obtain a higher output voltage. It is known that power cell based modular converters are advantageous for converter-fed generators with reduced stress on generator winding insulations. Therefore, there is a need for a wind power generation and conversion system with multi-phase wind turbine generator and power cell based modular converters to realize an efficient and reliable MVDC electrical system, in the range of 20 kV to 50 kV or higher, for offshore wind power plants. Additionally, there is a need for a wind farm electrical farm architecture with a high range MVDC collection system and high voltage transmission system. In particular, for HVAC-connected offshore wind farms there is a further need for collection system schemes which provide power electronics with middle frequency or high frequency transformers incorporated into DC/DC rectifiers with power cell based modular converter technology.